The present invention relates generally to electrical steel strip and more particularly to electrical steel strip for core laminations for rotating electrical machinery and transformers and to a method, for producing such a strip, employing a continuous strip casting procedure.
Rotating electrical machinery, such as electric motors or generators, and transformers have magnetic cores comprising laminations made from electrical steel sheet or strip having relatively good magnetic properties such as relatively low core loss and relatively high magnetic permeability. Steel is composed of a multitude of crystals or grains comprising unit cubes in which the atomic distribution is described as either body-centered-cubic (b.c.c.) or face centered cubic (f.c.c.). In a body centered cube, there are atoms at each of the eight corners of the cube and one atom in the center of the cube. In a face centered cube, there are atoms at each of the eight corners of the cube and atoms in the center of each of the six faces of the cube. There are multitudes of unit cubes in a crystal which, in turn, is typically microscopic in size.
At room temperature, a principal constituent of most steels is a solid phase called ferrite (also called alpha) which is a body-centered-cubic phase. Depending upon its composition, at elevated temperatures, the steel can undergo a phase change wherein some or all of the ferrite can change to another phase called austenite (also called gamma) which is face-centered-cubic. The higher the temperature, the greater the proportion of austenite. At still higher temperatures, and depending upon the composition of the steel, some or all of the austenite can change into a body-centered-cubic phase called delta. When such a steel solidifies from the molten state and cools to room temperature, the solid steel undergoes phase changes in the following sequence: delta (b.c.c.) to austenite (f.c.c.) to ferrite (b.c.c.). For purposes of this discussion, the delta and ferrite phases may be assumed to be the same, and they will hereinafter be referred to interchangeably as b.c.c. phase.
The crystalline texture of a steel strip is determined by the alignment, with the surface of the strip, of one of three relevant planes of a unit cube. A unit cube, whether it be b.c.c. or f.c.c, has one relevant plane, defined by a face of the cube, called the (100) plane. Another relevant plane of the unit cube extends diagonally in one direction from one cube edge to a diagonally opposite and parallel edge of the cube, and this plane is called the (110) plane. A third plane of the unit cube extends in two diagonal directions, from one corner of the cube to a diagonally opposite corner of the cube, and this third plane is called the (111) plane. The crystalline texture of a steel can be described by the alignment of one of these three planes of the unit cube with the surface of the steel strip and by the alignment of one of the direction vectors in the chosen plane with the sheet rolling direction. Depending upon the thermo-mechanical processing to which the steel strip has been subjected, in some cases a (100) plane of the unit cubes may lie in or be parallel to the surface of the strip; in other cases a (110) plane of the unit cubes may lie in or be parallel to the surface of the strip; and in still other cases a (111) plane of the unit cubes may lie in or be parallel to the surface of the strip.
For rotating electrical machinery, the best magnetic properties are obtained when a (100) plane of the unit cubes lies in or is parallel to the surface of the strip and these {100} planes are randomly oriented therein. Such a random orientation has its direction vector designated as &lt;uvw&gt;. A random orientation means that the {100} planes lying in the surface of the strip (or in a plane parallel thereto) are rotated about an edge of the unit cube extending perpendicularly to the strip surface, with the {100} planes of different cubes rotated different amounts.
Steel strip is normally the end product of a manufacturing method employing a number of thermo-mechanical processing steps. Typically, the steel strip has undergone hot rolling, cold rolling and annealing steps, some or all of which may have changed the crystalline texture of the strip. To produce the desired crystalline texture (in which a (100) plane of the unit cubes lies in the surface of the strip or in a plane parallel thereto, with these {100} planes being randomly oriented in that plane of the strip) has conventionally required a relatively complicated sequence of thermo-mechanical processing steps.
Another procedure for producing steel strip is known as continuous strip casting. In this procedure, molten steel is poured into the gap defined between two counter-rotating, cooled rolls having facing surfaces which define the casting mold. As the molten steel descends through the gap between the two rolls, the steel is cooled and solidified so that a solid strip of steel exits downwardly from the nip (i.e. the narrowest part of the gap) between the rolls. The thickness of the strip is determined by the width of the nip between the rolls. The solidified strip is then cooled to ambient temperature. Depending upon its composition, a steel undergoing continuous strip casting will, during solidification and cooling to ambient temperature, undergo a change in phase from b.c.c. to f.c.c. to b.c.c., and each such phase change disrupts the crystalline texture which prevailed during the preceding phase.
In addition to procedures employing two counter-rotating rolls, there are other continuous strip casting procedures employing a single roll or continuous belts; all of these procedures are known to those skilled in the art of continuous strip casting.
Examples of continuous strip casting procedures and equipment are described in Gerber, et al. U.S. Pat. No. 5,197,534, Gerber U.S. Pat. No. 5,279,350, Pareg [sic.] U.S. Pat. No. 4,936,374 and Praeg U.S. Pat. No. 5,251,685 and in Blazek, et al. U.S. application Ser. No. 07/928,848 filed Aug. 11, 1992, for example; and the disclosures of these patents and of said application are incorporated herein by reference.
An advantage of continuous strip casting is that it usually eliminates the need to perform hot rolling and associated other procedures. When reference is made herein to the elimination or absence of hot rolling, that does not exclude (a) the use of guide rollers for directing the hot strip as it exits the strip caster and moves downstream thereof or (b) the use of rolls to ameliorate small gage variations (i.e. non-uniformity of thickness) in the strip exiting the strip caster.